Pyrrolo[2,1-c][1,4]benzodiazepines (PBDs), a group of potent naturally occurring antitumour antibiotics from various Streptomyces species, are of considerable interest because of their ability to recognize and subsequently form covalent bonds to specific base sequence of double strand DNA (Dervan, P. B. Science 1989, 232, 464.; Hurley, L. H. J. Med. Chem. 1989, 32, 2027.; Thurston, D. E.; Thompson, A. S.Chem. Br. 1990, 26, 767). Well-known members of this group include anthramycin, DC-81, sibiromycin, tomamycin, chicamycin and neothramycin of A and B (Hurley, L. H. J. Antibiot. 1977, 30, 349.; Schimizu, K.; Kawamoto, I.; Tomita, F.; Morimoto, M.; Fujimoto, K. J. Antibiot 1982, 35, 992.; Lown, J. W.; Joshua, A. V. Biochem. Pharmacol. 1979, 28, 2017.; Thurston, D. E.; Bose, D. S. Chem. Rev. 1994, 94, 433.; Molina, P.; Diaz, I.; Tarraga, A. Tetrahedron 1995, 51, 5617.; Kamal, A.; Rao, N. V. Chem. Commun. 1996, 385.; Kamal, A.; Reddy, B. S. P.; Reddy, B. S. N. Tetrahedron Lett. 1996, 37, 6803). The cytotoxicity and antitumour activity of these agents are attributed to their property of sequence selective covalent binding to the N2 of guanine in the minor groove of duplex DNA via an acid-labile aminal bond to the electrophilic imine at the N10-C11 position (Kunimoto, S.; Masuda, T.; Kanbayashi, N.; Hamada, M.; Naganawa, H.; Miyamoto, M.; Takeuchi, T.; Unezawa, H. J. Antibiot, 1980, 33, 665.; Kohn, K. W. and Speous, C. L. J. Mol. Biol., 1970, 51, 551.; Hurley, L. H.; Gairpla, C. and Zmijewski, M. Biochem. Biophys. Acta., 1977, 475, 521.; Kaplan, D. J. and Hurley, L. H. Biochemistry, 1981, 20, 7572). The molecules have a right-handed twist, which allows them to follow the curvature of the minor groove of B-form double-stranded DNA spanning three base pairs. A recent development has been the linking of two PBD units through their C-8 positions to give bisfunctional-alkylating agents capable of cross-linking DNA (Thurston, D. E.; Bose, D. S.; Thomson, A. S.; Howard, P. W.; Leoni, A.; Croker, S. J.; Jenkins, T. C; Neidle, S. and Hurley, L. H. J. Org. Chem. 1996, 61, 8141).

Recently, PBD dimers have been developed that comprise of two C2-exo-methylene substituted DC-81 subunits tethered through their C-8 position via an inert propanedioxy linker (Gregson, S. J.; Howard, P. W.; Hartely, J. A.; Brooks, N. A.; Adams, L. J.; Jenkins, T. C; Kelland, L. R. and Thurston, D. E. J. Med. Chem. 2001, 44, 737). A non-cross-linking mixed imine-amide PBD dimers have been synthesized that have significant DNA binding ability and potent antitumour activity (Kamal, A.; Ramesh, G. Laxman, N.; Ramulu, P.; Srinivas, O.; Neelima, K.; Kondapi, A. K.; Srinu, V. B.; Nagarajaram, H. M. J. Med. Chem. 2002, 45, 4679). However, the clinical efficacy for these antibiotics is hindered by several limitations, such as poor water solubility, cardiotoxicity, development of drug resistance and metabolic inactivation. Due to the excellent activity of these molecules, there is need to develop novel derivatives which are devoid of above limitations.
Benzothiazoles are small synthetic molecules that contain a benzene ring fused to a thiazole ring. These simple molecules have shown remarkable antitumour properties and some of them are undergoing evaluation in clinical trials (Shi, D. -F.; Bradshaw, T. D.; Wrigley, S.; McCall, C. J.; Lelieveld, P.; Fichtner, I.; Stevens, M. F. G. J. Med. Chem. 1996, 39, 3375; Kashiyama, E.; Hutchinson, I.; Chua, M. -S.; Stinson, S. F.; Phillips, L. R.; Kaur, G.; Sausville, E. A.; Bradshaw, T. D.; Westwell, A. D.; Stevens, M. F. G. J. Med. Chem. 1999, 42, 4172; Hutchinson, I.; Chua, M. -S.; Browne, H. L; Trapani, V.; Bradshaw, T. D.; Westwell, A. D.; Stevens, M. F. G. J. Med. Chem. 2001, 44, 1446). Recently Westwell and coworkers have prepared a series of benzothiazole derivatives and evaluated for anticancer activity, One of these analogues has shown excellent anticancer activity (Mortimer, C. G.; Wells, G.; Crochard, J. -P.; Stone, E. L; Bradshaw, T. D.; Stevens, M. F. G.; Westwell, A. D. J. Med. Chem. 2006, 49, 179). The structurally related benzoxazoles have also been reported to possess anticancer activity (Kumar, D.; Jacob, M. R.; Reynold, M. B.; Kerwin, S. M. Bioorg. Med. Chem. 2002, 10, 3994; Gong, B.; Hong, F.; Kohm, C.; Bonham, L.; Klein, P. Bioorg. Med. Chem. Lett. 2004, 14, 1455). During earlier studies in this laboratory PBDs have been linked to benzothiazole or benzoxazole through alkane chain, which have shown promising anticancer activity. (Kamal, A.; Reddy K. S.; Ahmed K. M. N.; Shetti R. V. C. R. N. C International Publication No WO 2008/099416 A2). However, in the present invention the PBD and benzothiazole or benzoxazole moieties have been linked through piperazine moiety with alkyl side arms, instead simple alkane chain spacers. By incorporation of a piperazine moiety in the spacer these new hybrids not only exhibit enhanced in vitro activity but also remarkable DNA binding affinity for a number of this type of hybrids as illustrated in Table 1 and 2.
References may be made to patent “WO/2008/099416” wherein Benzothiazole or benzoxazole linked Pyrrolo[2,1-c][1,4] benzodiazepine hybrid as anticancer agent has been reported. References may be made to patent “U.S. Pat. No. 6,979,684” wherein Pyrrolo[2,1-c][1,4] benzodiazepine and naphthalimide are linked through piperazine moiety. The linking of pharmacophore like napthalimide and piperazine to pyrroloberizodiazepine can not be considered similar to a pharmacophore like benzothiazole as each pharmacophore is known for certain biological property. In the present investigation the benzothiaole and piperazine subunits are considered to be a different heterocyclic moieties compared to the earlier reported pharmacophores like napthalimide. Therefore, this structural variation of benzothiazole with piperazine moiety has been utilized for DNA binding aspect. Keeping this in mind, these new conjugates have been designed and synthesized to further improve the anticancer activity including the DNA binding affinity. The anticancer activity may not depend on the DNA binding activity. The DNA binding activity is only a biophysical aspect, therefore comparison of this data has no significance for the biological profile of the compounds. The new molecules of the present investigation exhibit potential anticancer activity. Moreover, one of the potent conjugate 9f (IICT-302) of this series has been evaluated for its in vivo efficacy studies (in scid male mice) against MCF-7 (breast cancer) and PC-3 (prostate cancer) xenografts by using adriamycin as a positive control. The in vivo efficacy study of 9f has exhibited less toxicity, good survival data and good RTV then control adriamycin indicating the potential use of these molecules in treating cancer.